How popular is Lennart?

Lennart is an uncommon given name for males. Lennart is also a unique surname for all people. (2000 U.S. Census)

Which version is better?

with its source form and related boy names. Leonardo(#103 the previous year), Leonard(#653), Lenny(#1145), Lennie, Lennard and Lenard are the prominent alternative forms of Lennart (not ranked) rated in the Top 2000. Other forms, like Lenn, are uncommon. These forms of Lennart were popular as baby names during 1920-1929 (average #1191) and are now significantly less popular (#1316, down 55.1%), with forms like Leonard becoming somewhat outmoded. Leonard has been the most well-known version, although Leonardo appears to be becoming a favorite too. (2015 U.S. SSA Records)